The present invention relates to the field of ultrasound imaging and, more particularly, to a system and method for increasing image acquisition speed.
Ultrasound imaging is a low-cost, safe, and mobile imaging modality that is widely used in clinical radiology. There are a number of modes in which ultrasound can be used to produce images of objects. The ultrasound transmitter may be placed on one side of the object and the sound transmitted through the object to the ultrasound receiver placed on the other side (“transmission mode”). With transmission mode methods, an image may be produced in which the brightness of each pixel is a function of the amplitude of the ultrasound that reaches the receiver (“attenuation” mode), or the brightness of each pixel is a function of the time required for the sound to reach the receiver (“time-of-flight” or “speed of sound” mode). In the alternative, the receiver may be positioned on the same side of the object as the transmitter and an image may be produced in which the brightness of each pixel is a function of the amplitude or time-of-flight of the ultrasound reflected from the object back to the receiver (“refraction”, “backscatter” or “echo” mode).
Ultrasonic transducers for medical applications are constructed from one or more piezoelectric elements sandwiched between a pair of electrodes. Such piezoelectric elements are typically constructed of lead zirconate titanate (PZT), polyvinylidene diflouride (PVDF), or PZT ceramic/polymer composite. The electrodes are connected to a voltage source, and when a voltage is applied, the piezoelectric elements change in size at a frequency corresponding to that of the applied voltage. When a voltage pulse is applied, the piezoelectric element emits an ultrasonic wave into the media to which it is coupled at the frequencies contained in the excitation pulse. Conversely, when an ultrasonic wave strikes the piezoelectric element, the element produces a corresponding voltage across its electrodes. Typically, the front of the element is covered with an acoustic matching layer that improves the coupling with the media in which the ultrasonic waves propagate. In addition, a backing material is disposed to the rear of the piezoelectric element to absorb ultrasonic waves that emerge from the back side of the element so that they do not interfere. A number of such ultrasonic transducer constructions are disclosed in U.S. Pat. Nos. 4,217,684; 4,425,525; 4,441,503; 4,470,305 and 4,569,231.
When used for ultrasound imaging, the transducer typically has a number of piezoelectric elements arranged in an array and driven with separate voltages (apodizing). By controlling the time delay (or phase) and amplitude of the applied voltages, the ultrasonic waves produced by the piezoelectric elements (transmission mode) combine to produce a net ultrasonic wave focused at a selected point. By controlling the time delay and amplitude of the applied voltages, this focal point can be moved in a plane to scan the subject.
The same principles apply when the transducer is employed to receive the reflected sound (receiver mode). That is, the voltages produced at the transducer elements in the array are summed together such that the net signal is indicative of the sound reflected from a single focal point in the subject. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the echo signal received by each transducer array element.
As indicated above, there are a number of electronic methods for performing a scan using a transducer having an array of separately operable elements. These methods include linear array systems and phased array systems.
A linear array system includes a transducer having a large number of elements disposed in a line. A small group of elements are energized to produce an ultrasonic beam that travels away from the transducer, perpendicular to its surface. The group of energized elements is translated along the length of the transducer during the scan to produce a corresponding series of beams that produce echo signals from a two-dimensional region in the subject. To focus each beam that is produced, the pulsing of the inner elements in each energized group is delayed with respect to the pulsing of the outer elements. The time delays determine the depth of focus which can be changed during scanning. The same delay factors are applied when receiving the echo signals to provide dynamic focusing during the receive mode. A number of such linear array systems are disclosed in U.S. Pat. Nos. 3,881,466; 4,550,606 and 5,097,709.
The second common form of ultrasonic imaging is referred to as “phased array sector scanning”, or “PASS”. Such a scan is comprised of a series of measurements in which all of the elements of a transducer array are used to transmit a steered ultrasonic beam. The system then switches to receive mode after a short time interval, and the reflected ultrasonic wave is received by all of the transducer elements. Typically, the transmission and reception are steered in the same direction (θ) during each measurement to acquire data from a series of points along a scan line. The receiver is dynamically focused at a succession of ranges (R) along the scan line as the reflected ultrasonic waves are received. A series of measurements are made at successive steering angles (θ) to scan a pie-shaped sector of the subject. The time required to conduct the entire scan is a function of the time required to make each measurement and the number of measurements required to cover the entire region of interest at the desired resolution and signal-to-noise ratio. For example, a total of 128 scan lines may be acquired over a 90° sector, with each scan line being steered in increments of 0.70°. A number of such ultrasonic imaging systems are disclosed in U.S. Pat. Nos. 4,155,258; 4,155,260; 4,154,113; 4,155,259; 4,180,790; 4,470,303; 4,662,223; 4,669,314 and 4,809,184.
The same scanning methods may be used to acquire a three-dimensional image of the subject. The transducer in such case is a two-dimensional array of elements which steer a beam throughout a volume of interest or linearly scan a plurality of adjacent two-dimensional slices.
Ultrasound imaging, which can generate images as fast as the human eye can see them, has a high temporal resolution compared to other imaging modalities. However, image quality is generally sacrificed to achieve these high frame rates and imaging parameters, such as the number of lines per image and the maximum depth, must be adjusted in consequence. Maintaining high frame rates also prevents the practical use of more elaborate imaging techniques, for example, techniques providing improved spatial coverage or multidimensional images. A method able to increase imaging speed would improve the feasibility and utility of such scanning methods. While current frame rates are arguably sufficient in diagnostic ultrasound, faster imaging techniques would be valuable, not necessarily to provide increased frame rates, but to provide more elaborate and improved images while keeping frame rates unchanged.
Synthetic aperture imaging is a fast imaging technique that allows the generation of an image after every transmit event and involves firing a single element of a transducer while receiving signal from all elements. Although fast, synthetic aperture imaging is adversely affected by reduced signal-to-noise ratio (SNR) and increased artifact content. Modifications have been proposed to alleviate these problems, for example, extending acquisition over multiple transmit events, employing several elements to create a focus point that acts as a virtual element with increased power, and firing multiple elements at once (either physical or virtual) using voltage waveforms that are later discriminated during image reconstruction. These waveforms may be designed through techniques such as frequency hopping, frequency division, and the generation of pseudo-random sequences. However, these transmit schemes do not provide adequate image quality when compared to traditional transmit beamforming strategies, especially when imaging objects having the complexity of typical biological systems.
Faster imaging can be performed using receive beamforming, which features abilities to discriminate between echo signal components from different simultaneously-transmitted ultrasound beams and thus allows faster image acquisition. However, the performance of receive beamforming using such techniques is strongly dependent on the precise shape of the transmitted waveforms and, as a result, performance may vary significantly between different imaging situations. In many cases, images produced using this technique may include excessive artifact levels.
It would therefore be desirable to have a system and method for accelerated ultrasound method that provides faster image acquisition without producing significant levels of artifacts. Such a method would allow improvements in image quality and the use of more elaborate scanning techniques, for example, multi-plane imaging, without accompanying reductions in frame rate.